Graphene is a kind of two-dimensional single-layer carbon nanostructured material constructed by the bottom up assembly of sp2 carbon atoms, which has great application potentials in the fields of electronic devices, sensors, nanocomposite materials and electrochemical energy storage because of its excellent conductivity, thermal conductivity, mechanical properties and chemical stability.
Porous graphene materials, possessing both the high conductivity of the graphene sheet structure and rich pore structure, show significant application values in electrochemical energy storage, especially in supercapacitors based on ion adsorption principle. At present, the preparation methods of porous graphene mainly includes chemical activation, template synthesis and carbon thermal reduction. Therein, the chemical activation method involves the activating etching of graphite or graphite oxide by KOH, H3PO4, ZnCl2 or other activation agents, wherein the graphite or graphite oxide are pre-exfoliated by microwave or other chemical means. The literature [Science, vol. 332, 1537, (2011)] provided a method for preparing microporous graphene by KOH activation of microwave-exfoliated graphene oxide. The resulting microporous graphene has a surface area of 3100 m2/g and exhibits excellent organic supercapacitor performance. The template synthesis of porous graphene is based on a deposition process in which carbon source pass through the templates (MgO, ZnS, SiO2, Al2O3 and so on) to form graphene layers. Subsequent template removal process gives porous graphene materials. For example, porous graphene materials with various pore features can be obtained based on catalytic deposition process which employs MgO (flaky, spherical, columnar) as the substrate template and small molecule hydrocarbons (CH4, C2H4) as the gas-phase carbon source [Nat. Commun, 5, 3410 (2014)]. In addition, carbon thermal reaction can be also applied to prepare porous graphene. Such process is to use graphite oxide as raw material and oxygenated metal salts (such as Na2MoO4, Na2WO4, Na3VO4, NaAlO2, Na2SnO3 and K2TiO3) as etchant in which a high temperature (650° C.) reduction reaction and subsequent pickling of metal oxide yield the porous graphene materials with pore size of 1 to 50 nm. [Nat. Commun, 5, 4716 (2014)]. However, it should be noted that all the above-mentioned preparation methods for porous graphene material suffer from bottlenecks including high cost of raw materials, time consuming procedures and mass production difficulty.
Thus, the development of low-cost, large-scale preparation methods of porous graphene material is of great significance. To achieve this purpose, it is critical to seek raw materials with low-cost and large reserves. The internal structure of coal contains a large amount of natural graphite-like structure with aromatic hydrocarbons and polyarylene as the basic unit. It has long been regarded as one of the important raw materials for large-scale and low-cost preparation of porous carbon materials. At present, the production of coal based porous carbon material from is mainly based on the carbonization and activation (physical activation or chemical activation) of bituminous coal or anthracite. Characteristics of coal, activation agent type and activation condition are the key factors influencing the pore structure of resulting porous carbons. However, the carbon framework in the coal-based activated carbon material prepared by current strategies is mainly amorphous structure, and the pores created by the physical or chemical activation method are poor (specific surface area <1500 m2/g). Moreover, the heteroatom content is high, which limits its application in the efficient adsorption of gas molecules and electrochemical energy storage. Compared to other types of coal, lignite in China has a large reserve. Especially in Chinese Xinjiang Zhungeer eastern, low-grade coal resources (quasi-East lignite) has been recently found with a forecast reserves of 390 billion tons. On the one hand, lignite due to the lower degree of coalification, higher alkali metal and moisture content, greatly limits its large-scale application in the field of coal-fired power generation. On the other hand, lignite has significant advantages as raw material for preparing porous carbon materials: (1) high content of lignite volatile content, which is favorable for the formation of well-developed pore structure during pyrolysis process; (2) low degree of coalification, which makes lignite have a high reactivity and easy to convert to porous carbon materials by the regulation of carbon structure evolution and pore formation process. At present, lignite is mainly used to prepare low-quality activated coke for water treatment and removal of coal-fired flue gas. A small amount of research has been reported to prepare activated carbon with potassium-containing activation agents. Although the activated carbon with high specific surface area could be obtained, the porous carbon material was still amorphous and the heteroatom content was high.
The present invention provides a method for preparing porous graphene materials, which is low-cost and easy for large-scale industrial production. In particular, lignite with special degree of coalification and structural characteristics is used as raw material; the porous graphene material is prepared by one-step chemical activation process in which the initial the graphene-like structures and the catalytic metal compositions in coal are two factors for the formation of graphene structures. Such a preparation method of graphene material is simple, low cost and easy to be large-scale and batch production. The porous graphene prepared by the method has the advantages of well-developed pore structure, controlled specific surface area and high purity of carbon structure, which make it has great application potentials in the fields of electrochemical energy storage (Double layer supercapacitor electrode material, lithium ion capacitor cathode material) and gas adsorption (CO2 adsorption, CH4 adsorption).